The Mekong continental shelf: The primary sink for deltaic sediment particles and their passengers

Abstract

The Mekong River discharges into the Vietnamese East Sea and forms a subaqueous deposit known as a clinoform, the region of greatest sediment accumulation for the deltaic system. The peak discharge from the Mekong River is linked to seasonal monsoon conditions and occurs from August to November when ocean conditions are relatively quiescent. The river plume carries sediment to the clinoform where it is deposited. Subsequently (January to April), the coupling of energetic surface waves, tidal currents, and wind-driven currents resuspends much of the newly deposited shelf sediment and transports it landward and southwestward. This pathway displaces about two-thirds of the sediment to nourish distal coastal regions along the Ca Mau Peninsula. Throughout the year, the seaward flow of the Mekong surface plume causes estuarine circulation to draw much underlying ocean water landward onto the shelf (more than twice the volume of Mekong freshwater discharge). This process allows dissolved chemical components to become attached (i.e., adsorbed) to surfaces of suspended particles. The sediment accumulating on the seabed buries a broad array of these components from sources in the adjacent ocean water as well as the river drainage basin. The ocean portion of the Mekong River dispersal system involves an intricate coupling of temporally variable processes (e.g., river discharge, coastal winds, and along-shelf currents). Future alterations to the river source (e.g., due to damming) and to the ocean sink (e.g., due to climate change) will likely affect this coupling and modify the processes occurring on the Mekong continental shelf.